In recent years, there is known a corresponding point searching method such that a plurality of standard images having resolutions different from each other and a plurality of reference images having resolutions different from each other are hierarchically generated respectively from a standard image and a reference image, a search region is set in an upper layer based on a corresponding point searched in a lower layer, and a corresponding point is searched in the set search region. Such a corresponding point searching method is called as a multi-resolution strategy.
For instance, patent literature 1 discloses a technology, in which a degree of reliability for evaluating the reliability of a corresponding point is calculated, a corresponding point having a low degree of reliability is changed to another corresponding point, and an initial search position in a layer immediately higher than a target layer is determined based on the new corresponding point.
In the aforementioned corresponding point searching method utilizing a multi-resolution strategy, a target point is set in a standard image, a correlation value with respect to the target point is obtained at each of positions in a search region set in a reference image, and a position having a largest correlation value among the correlation values is determined as a corresponding point. The correlation values are calculated with use of e.g. an SAD method or a POC method.
The aforementioned corresponding point searching method is carried out, based on the premise that a subject is present at a distance substantially equal to the distance from a camera within a search region.
However, there is a case that plural subjects having subject distances different from the distance from the camera are present in a search region. In such a case, plural sharp correlation peaks may appear within the search region. If such plural sharp correlation peaks appear, it is difficult to determine which one of the correlation peaks represents a true corresponding point.